A storage system typically comprises one or more storage devices into which information may be entered, and from which information may be obtained, as desired. The storage system includes a storage operating system that functionally organizes the system by, inter alia, invoking storage operations in support of a storage service implemented by the system. The storage system may be implemented in accordance with a variety of storage architectures including, but not limited to, a network-attached storage environment, a storage area network and a disk assembly directly attached to a client or host computer. The storage devices are typically disk drives organized as a disk array, wherein the term “disk” commonly describes a self-contained rotating magnetic media storage device. The term disk in this context is synonymous with hard disk drive (HDD) or direct access storage device (DASD).
Storage of information on the disk array is preferably implemented as one or more storage “volumes” of physical disks, defining an overall logical arrangement of disk space. The disks within a volume are typically organized as one or more groups, wherein each group may be operated as a Redundant Array of Independent (or Inexpensive) Disks (RAID). Most RAID implementations enhance the reliability/integrity of data storage through the redundant writing of data “stripes” across a given number of physical disks in the RAID group, and the appropriate storing of redundant information (parity) with respect to the striped data. The physical disks of each RAID group may include disks configure to store striped data (i.e., data disks) and disks configure to store parity for the data (i.e., parity disks). The parity may thereafter be retrieved to enable recovery of data lost when a disk fails. The term “RAID” and its various implementations are well-known and disclosed in A Case for Redundant Arrays of Inexpensive Disks (RAID), by D. A. Patterson, G. A. Gibson and R. H. Katz, Proceedings of the International Conference on Management of Data (SIGMOD), June 1988.
The storage operating system of the storage system may implement a high-level module, such as a file system, to logically organize the information stored on the disks as a hierarchical structure of directories, files and blocks. For example, each “on-disk” file may be implemented as set of data structures, i.e., disk blocks, configured to store information, such as the actual data for the file. These data blocks are organized within a volume block number (vbn) space that is maintained by the file system. The file system organizes the data blocks within the vbn space as a “logical volume”; each logical volume may be, although is not necessarily, associated with its own file system. The file system typically consists of a contiguous range of vbns from zero to n−1, for a file system of size n blocks.
A known type of file system is a write-anywhere file system that does not over-write data on disks. If a data block is retrieved (read) from disk into a memory of the storage system and “dirtied” (i.e., updated or modified) with new data, the data block is thereafter stored (written) to a new location on disk to optimize write performance. A write-anywhere file system may initially assume an optimal layout such that the data is substantially contiguously arranged on disks. The optimal disk layout results in efficient access operations, particularly for sequential read operations, directed to the disks. An example of a write-anywhere file system that is configure to operate on a storage system is the Write Anywhere File Layout (WAFL™) file system available from Network Appliance, Inc., Sunnyvale, Calif.
The storage operating system may further implement a storage module, such as a RAID system, that manages the storage and retrieval of the information to and from the disks in accordance with input/output (I/O) operations. The RAID system is also responsible for parity operations in the storage system. Note that the file system only “sees” the data disks within its vbn space; the parity disks are “hidden” from the file system and, thus, are only visible to the RAID system. The RAID system typically organizes the RAID groups into one large “physical” disk (i.e., a physical volume), such that the disk blocks are concatenated across all disks of all RAID groups. The logical volume maintained by the file system is then “disposed over” (spread over) the physical volume maintained by the RAID system.
The storage system may be configure to operate according to a client/server model of information delivery to thereby allow many clients to access the directories, files and blocks stored on the system. In this model, the client may comprise an application, such as a database application, executing on a computer that “connects” to the storage system over a computer network, such as a point-to-point link, shared local area network, wide area network or virtual private network implemented over a public network, such as the Internet. Each client may request the services of the file system by issuing file system protocol messages (in the form of packets) to the storage system over the network. By supporting a plurality of file system protocols, such as the conventional Common Internet File System (CIFS) and the Network File System (NFS) protocols, the utility of the storage system is enhanced.
When accessing a block of a file in response to servicing a client request, the file system specifies a vbn that is translated at the file system/RAID system boundary into a disk block number (dbn) location on a particular disk (disk, dbn) within a RAID group of the physical volume. It should be noted that a client request is typically directed to a specific file block number (fbn), which represents an offset into a particular file. For example, if a file system is using 4 KB blocks, fbn 6 of a file represents a block of data starting 24 KB into the file and extending to 28 KB, where fbn 7 begins. The fbn is converted to an appropriate vbn by the file system. Each block in the vbn space and in the dbn space is typically fixed, e.g., 4 k bytes (kB), in size; accordingly, there is typically a one-to-one mapping between the information stored on the disks in the dbn space and the information organized by the file system in the vbn space. The (disk, dbn) location specified by the RAID system is further translated by a disk driver system of the storage operating system into a plurality of sectors (e.g., a 4 kB block with a RAID header translates to 8 or 9 disk sectors of 512 or 520 bytes) on the specified disk.
The requested block is then retrieved from disk and stored in a buffer cache of the memory as part of a buffer tree of the file. The buffer tree is an internal representation of blocks for a file stored in the buffer cache and maintained by the file system. Broadly stated, the buffer tree has an inode at the root (top-level) of the file. An inode is a data structure used to store information, such as metadata, about a file, whereas the data blocks are structures used to store the actual data for the file. The information contained in an inode may include, e.g., ownership of the file, access permission for the file, size of the file, file type and references to locations on disk of the data blocks for the file. The references to the locations of the file data are provided by pointers, which may further reference indirect blocks that, in turn, reference the data blocks, depending upon the quantity of data in the file. Each pointer may be embodied as a vbn to facilitate efficiency among the file system and the RAID system when accessing the data on disks.
The RAID system maintains information about the geometry of the underlying physical disks (e.g., the number of blocks in each disk) in raid labels stored on the disks. The RAID system provides the disk geometry information to the file system for use when creating and maintaining the vbn-to-disk,dbn mappings used to perform write allocation operations and to translate vbns to disk locations for read operations. Block allocation data structures, such as an active map, a snapmap, a space map and a summary map, are data structures that describe block usage within the file system, such as the write-anywhere file system. These mapping data structures are independent of the geometry and are used by a write allocator of the file system as existing infrastructure for the logical volume. Examples of the block allocation data structures are described in U.S. Patent Application Publication No. US2002/0083037 A1, titled Instant Snapshot, by Blake Lewis et al. and published on Jun. 27, 2002, which application is hereby incorporated by reference.
The write-anywhere file system typically performs write allocation of blocks in a logical volume in response to an event in the file system (e.g., dirtying of the blocks in a file). When write allocating, the file system uses the block allocation data structures to select free blocks within its vbn space to which to write the dirty blocks. The selected blocks are generally in the same positions along the disks for each RAID group (i.e., within a stripe) so as to optimize use of the parity disks. Stripes of positional blocks may vary among other RAID groups to, e.g., allow overlapping of parity update operations. When write allocating, the file system traverses a small portion of each disk (corresponding to a few blocks in depth within each disk) to essentially “lay down” a plurality of stripes per RAID group. In particular, the file system chooses vbns that are on the same stripe per RAID group during write allocation using the vbn-to-disk,dbn mappings.
The on-disk structure of the file system is comprised of a number of entities of discrete data structures organized with appropriate pointers to layers beneath one entity. A storage system may utilize one physical volume, wherein the volume comprises of a number of physical disks associated in an arrangement, such as a RAID group for improved data protection. The physical volume utilizes physical volume block numbers (pvbns) within indirect blocks and inodes to point to other data structures within the on-disk structure of the file system.
FIG. 1 is a schematic block diagram of an exemplary on-disk storage arrangement 100 of a conventional physical volume of a file system. The on-disk storage arrangement 100 comprises a volinfo block 102 that contains pointers to various fsinfo blocks including fsinfo block 105 representing the active file system, as well as fsinfo blocks 110 and 115 representing various snapshots, or persistent consistency point images (PCPIs) associated with the active file system. It should be noted that “snapshot” is a trademark of Network Appliance, Inc. and is used for purposes of this patent to designate a persistent consistency point (CP) image. A PCPI is a space conservative, point-in-time read-only image of data accessible by name that provides a consistent image of that data (such as a storage system) at some previous time. More particularly, a PCPI is a point-in-time representation of a storage element, such as an active file system, volume, virtual file system, file or database, stored on a storage device (e.g., on disk) or other persistent memory and having a name or other identifier that distinguishes it from other PCPIs taken at other points in time. A PCPI can also include other information (metadata) about the active file system at the particular point in time for which the image is taken. The terms “PCPI” and “snapshot” may be used interchangeably through out this patent without derogation of Network Appliance's trademark rights.
The volinfo 102 is illustratively located at vbns 1 and 2 or, in alternate embodiments, at another predetermined location on disk. Each fsinfo block 105, 110, 115 is illustratively contained within an fsinfo file, the contents of which comprise the fsinfo block. In this example, the fsinfo block 105 for the active file system includes the inodes of the inode file for the active file system 120. The inode file for the active file system 120 includes further inodes for an active map 125, a summary map 130, a space map 135, a root directory 140 and a hidden metadata directory 145. Each additional fsinfo block, for example, fsinfo blocks 110 and 115, that is associated with a PCPI includes the inode of the inode file for the PCPI, which in turn includes appropriate inodes for active maps and the like (not shown) for the specific PCPI.
Other possible on-disk structures may be used with a storage system. For example, a volume may be modified so as to comprise an aggregate having a plurality of virtual volumes therein. Aggregates and virtual volumes are further described in U.S. patent application Ser. No. 10/836,817, issued as U.S. Pat. No. 7,409,494, entitled EXTENSION OF WRITE ANYWHERE FILE SYSTEM LAYOUT, by John K. Edwards, et al. In such a storage system, the file system utilizes and interprets pointers contained within the various on-disk structures, including the volume information and fsinfo blocks according to a predetermined virtual volume format. However, there exists no adequate technique for permitting a storage system to support volumes having different formats simultaneously. That is, the storage system is typically “hard-coded” to utilize one type of volume format and to utilize a differing format, all volumes associated with the storage system need to be modified. Thus, if a storage system utilizes a conventional physical volume and a set of disks comprising an aggregate are connected thereto, the storage system will incorrectly interpret the data contained within the aggregate's disks due to the differing format of pointers contained therein.